fileWriteStreaming

fn fileWriteStreaming(
    userdata: ?*anyopaque,
    file: File,
    header: []const u8,
    data: []const []const u8,
    splat: usize,
) File.Writer.Error!usize {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    _ = t;

    if (is_windows) {
        const buffer = windowsWriteBuffer(header, data, splat);
        if (buffer.len == 0) return 0;
        return fileWriteStreamingWindows(file, buffer);
    }

    var iovecs: [max_iovecs_len]posix.iovec_const = undefined;
    var iovlen: iovlen_t = 0;
    addBuf(&iovecs, &iovlen, header);
    for (data[0 .. data.len - 1]) |bytes| addBuf(&iovecs, &iovlen, bytes);
    const pattern = data[data.len - 1];

    var splat_backup_buffer: [splat_buffer_size]u8 = undefined;
    if (iovecs.len - iovlen != 0) switch (splat) {
        0 => {},
        1 => addBuf(&iovecs, &iovlen, pattern),
        else => switch (pattern.len) {
            0 => {},
            1 => {
                const splat_buffer = &splat_backup_buffer;
                const memset_len = @min(splat_buffer.len, splat);
                const buf = splat_buffer[0..memset_len];
                @memset(buf, pattern[0]);
                addBuf(&iovecs, &iovlen, buf);
                var remaining_splat = splat - buf.len;
                while (remaining_splat > splat_buffer.len and iovecs.len - iovlen != 0) {
                    assert(buf.len == splat_buffer.len);
                    addBuf(&iovecs, &iovlen, splat_buffer);
                    remaining_splat -= splat_buffer.len;
                }
                addBuf(&iovecs, &iovlen, splat_buffer[0..@min(remaining_splat, splat_buffer.len)]);
            },
            else => for (0..@min(splat, iovecs.len - iovlen)) |_| {
                addBuf(&iovecs, &iovlen, pattern);
            },
        },
    };

    if (iovlen == 0) return 0;

    if (native_os == .wasi and !builtin.link_libc) {
        var n_written: usize = undefined;
        const syscall: Syscall = try .start();
        while (true) {
            switch (std.os.wasi.fd_write(file.handle, &iovecs, iovlen, &n_written)) {
                .SUCCESS => {
                    syscall.finish();
                    return n_written;
                },
                .INTR => {
                    try syscall.checkCancel();
                    continue;
                },
                else => |e| {
                    syscall.finish();
                    switch (e) {
                        .INVAL => |err| return errnoBug(err),
                        .FAULT => |err| return errnoBug(err),
                        .AGAIN => |err| return errnoBug(err),
                        .BADF => return error.NotOpenForWriting, // can be a race condition.
                        .DESTADDRREQ => |err| return errnoBug(err), // `connect` was never called.
                        .DQUOT => return error.DiskQuota,
                        .FBIG => return error.FileTooBig,
                        .IO => return error.InputOutput,
                        .NOSPC => return error.NoSpaceLeft,
                        .PERM => return error.PermissionDenied,
                        .PIPE => return error.BrokenPipe,
                        .NOTCAPABLE => return error.AccessDenied,
                        else => |err| return posix.unexpectedErrno(err),
                    }
                },
            }
        }
    }

    const syscall: Syscall = try .start();
    while (true) {
        const rc = posix.system.writev(file.handle, &iovecs, @intCast(iovlen));
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                return @intCast(rc);
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            else => |e| {
                syscall.finish();
                switch (e) {
                    .INVAL => |err| return errnoBug(err),
                    .FAULT => |err| return errnoBug(err),
                    .AGAIN => return error.WouldBlock,
                    .BADF => return error.NotOpenForWriting, // Can be a race condition.
                    .DESTADDRREQ => |err| return errnoBug(err), // `connect` was never called.
                    .DQUOT => return error.DiskQuota,
                    .FBIG => return error.FileTooBig,
                    .IO => return error.InputOutput,
                    .NOSPC => return error.NoSpaceLeft,
                    .PERM => return error.PermissionDenied,
                    .PIPE => return error.BrokenPipe,
                    .CONNRESET => |err| return errnoBug(err), // Not a socket handle.
                    .BUSY => return error.DeviceBusy,
                    .ACCES => return error.AccessDenied,
                    else => |err| return posix.unexpectedErrno(err),
                }
            },
        }
    }
}